Starting Piano music with acciaccatura on upper voice

[Alberto Simões]

Hi

I am starting a piano music this way:

upper = \relative c'' {
  \clef treble
  \key g \major
  \time 4/4

  \acciaccatura ais8 b8. ....
}

and the lower voice as

lower = \relative c {
  \clef bass
  \key g \major
  \time 4/4
  fis8
}

but the lower staff starts with a treble clef followed by the bass clef
with the key signature.
This is quite weird

What am I doing wrong?
Thanks